However, evidence for the existence of DM on large scales does not directly constrain its mass range or interaction strength, but rather its mass density (about 0.3 times the critical density \(\rho _c\)  on large scales, and around 0.4 GeV cm\(^{-3}\)  in the Solar neighbourhood), and interaction rate with the Standard Model (SM) of particle physics.
As the viable parameter space for WIMP-like dark matter shrinks following null measurements from sensitive direct detection experiments developed over the past decades, the scope of DM direct detection searches has broadened. Candidates include sub-GeV dark matter [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, superheavy dark matter [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, [19]} and macro dark matter [20]}, [21]}, [22]}, [23]}. However, the WIMP family still includes many viable candidates that remain out of reach from the current generation of experiments. In particlular, Inelastic (I)DM particles, with \(O\) (keV) or larger mass splittings are expected in a variety of dark matter models [24]}, [25]}, [26]}, [27]}, [28]}, [29]}, [30]}, [31]}, [32]}, [33]}. Multi-state DM has been invoked in possible explanations of the 511 keV gamma ray excess observed in the galactic center [34]}, [35]}, [36]}, [37]}, [38]} and the DAMA/LIBRA annual modulation signals [25]}, [40]}, [41]}, [41]}, [43]}, [44]}. Recently, interest in inelastic dark matter has also been revived to interpret the XENON1T electron recoil data [45]} with exothermic scattering or luminous dark matter [46]}, [47]}, [48]}, [49]}, [50]}, [51]}, [52]}, [53]}, [54]}, [55]}, [56]}, [57]}, [58]}, [59]}, [60]}, [61]}.
